Implantable medical devices are required to self-monitor their status for a variety of reasons. One status requirement is the battery voltage level to determine device lifetime left. Typically, the voltage on a lithium-manganese-dioxide battery, as with other battery chemistries, is fairly stable as long as the charge remaining in the battery is above a certain limit. The battery voltage ramps down rapidly, however, once the charge has been depleted beyond this limit. This makes most of the lower voltage range of a battery impractical for use, as very little battery life remains thereafter. As a result, only an upper range of a battery voltage is worth monitoring since the battery would be considered consumed or “dead” below the charge-limited voltage level. Since an analog-to-digital converter (ADC) with a limited number of bits is usually used to further process the measured voltage, limiting the voltage range of interest also allows for better resolution of the usable voltage instead of having the ADC operate over the entire battery voltage.
A common approach is to introduce a level shifting device, such as a zener diode, to eliminate the unneeded bottom voltage range from being measured. However, zener diodes provide a highly variable voltage level at low currents. To maintain a stable voltage, one must drive a zener diode with enough current (100 micro-amps or more) to reach a stable voltage portion on the zener's current-voltage curve. As the voltage across the battery being measured drops, however, this current level can also drop, again introducing variability. To avoid variability, added current margin is usually provided to keep the zener diode past the knee voltage.
In the field of implantable defibrillators, battery characterization must also accommodate the periodic reformation of high power capacitors for use in therapy delivery. Periodic reformation of therapy capacitors occurs at intervals (often one to three times yearly) to ensure the capacitors maintain desirable operating characteristics. Capacitor reformation is performed by putting the device in a high current state for a short period of time to charge the capacitors to a high voltage (several hundred volts or more). This high current operation depresses battery voltage for several days after it is performed in batteries commonly used in implantable systems.
To accommodate the depressed battery voltage after high current operations, designs may withhold battery measurement for a period of time (a number of days). However, to more effectively monitor battery status, it may be desirable to have, first, a narrow upper range of measurement for daily use, and a second broader range of measurement for use following the capacitor reformation.
Lower current consuming monitoring circuits with a high degree of stability are hence desired, which can allow for multiple voltage ranges to be accurately monitored.